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Tsirkin" To: zhenwei pi Cc: jasowang@redhat.com, virtualization@lists.linux-foundation.org, linux-kernel@vger.kernel.org, xuanzhuo@linux.alibaba.com, Joerg Roedel Subject: Re: [PATCH] virtio_ring: use u32 for virtio_max_dma_size Message-ID: <20230704020935-mutt-send-email-mst@kernel.org> References: <20230510025437.377807-1-pizhenwei@bytedance.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230510025437.377807-1-pizhenwei@bytedance.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 10, 2023 at 10:54:37AM +0800, zhenwei pi wrote: > Both split ring and packed ring use 32bits to describe the length of > a descriptor: see struct vring_desc and struct vring_packed_desc. > This means the max segment size supported by virtio is U32_MAX. > > An example of virtio_max_dma_size in virtio_blk.c: > u32 v, max_size; > > max_size = virtio_max_dma_size(vdev); -> implicit convert > err = virtio_cread_feature(vdev, VIRTIO_BLK_F_SIZE_MAX, > struct virtio_blk_config, size_max, &v); > max_size = min(max_size, v); > > There is a risk during implicit convert here, once virtio_max_dma_size > returns 4G, max_size becomes 0. > > Fixes: e6d6dd6c875e ("virtio: Introduce virtio_max_dma_size()") > Cc: Joerg Roedel > Signed-off-by: zhenwei pi > --- > drivers/virtio/virtio_ring.c | 12 ++++++++---- > include/linux/virtio.h | 2 +- > 2 files changed, 9 insertions(+), 5 deletions(-) > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c > index c5310eaf8b46..55cfecf030a1 100644 > --- a/drivers/virtio/virtio_ring.c > +++ b/drivers/virtio/virtio_ring.c > @@ -289,12 +289,16 @@ static bool vring_use_dma_api(const struct virtio_device *vdev) > return false; > } > > -size_t virtio_max_dma_size(const struct virtio_device *vdev) > +u32 virtio_max_dma_size(const struct virtio_device *vdev) > { > - size_t max_segment_size = SIZE_MAX; > + u32 max_segment_size = U32_MAX; > > - if (vring_use_dma_api(vdev)) > - max_segment_size = dma_max_mapping_size(vdev->dev.parent); > + if (vring_use_dma_api(vdev)) { > + size_t max_dma_size = dma_max_mapping_size(vdev->dev.parent); > + > + if (max_dma_size < max_segment_size) > + max_segment_size = max_dma_size; > + } > > return max_segment_size; > } Took a while for me to get this, it's confusing. I think the issue is really in virtio blk, so I would just change max_size there to size_t and be done with it. > diff --git a/include/linux/virtio.h b/include/linux/virtio.h > index b93238db94e3..1a605f408329 100644 > --- a/include/linux/virtio.h > +++ b/include/linux/virtio.h > @@ -147,7 +147,7 @@ int virtio_device_restore(struct virtio_device *dev); > #endif > void virtio_reset_device(struct virtio_device *dev); > > -size_t virtio_max_dma_size(const struct virtio_device *vdev); > +u32 virtio_max_dma_size(const struct virtio_device *vdev); > > #define virtio_device_for_each_vq(vdev, vq) \ > list_for_each_entry(vq, &vdev->vqs, list) > -- > 2.20.1